Heretofore, the propulsion water intake for marine jet drives have been a compromise, since the intake of fixed cross sectional capacity has been made large enough for low speed performance and yet small enough for high speed perrormance. This compromise is not conducive to all-out racing or optimum all around performance, because the boat must be accelerated through the low speed barrier with the expectation of subsequent performance at high speed. Although intake scoops have been made to vary in size, jet boat drives have remained restricted in high speed performance, it being a general object of this invention to provide means by which jet driven boats may break away from restricted high speed performance and compete favorably with propeller driven boats that have a substantially high speed restriction.
There are two principle types of intakes for withdrawing water from the slipstream passing along the hull of a boat, the flush opening type and the scoop or ram type. The former is generally employed for the induction of water into jet propulsion drives, and the latter has been proposed but therefor in abortive configurations. The flush intake is efficient enough for moderate performance boats, it has low drag and does not interfere with boat handling. On the other hand, the scoop or ram type of intake is desirable for high speed operation as it provides high intake pressures so that the pump impeller will continue to operate on its performance curve at high speeds. Accordingly, it is an object of this invention to adapt a scoop or ram intake to the flush intake of a boat bottom in combination with a jet drive pump unit.
Experience teaches that larger intake area is required for low speed boat operation, while smaller intake area is required for high speed boat operation. Therefore, a variation in intake area is required for high performance throughout both the low and high speed ranges. For example, if performance is insufficient in the low speed range, the high speed range may not be reached. Accordingly, it is an object of this invention to provide an automatically variable volume propulsion water intake for hydrojet boat drives, having a restrictive ram opening for high speed operation and having a flush secondary opening augmenting the intake water for low speed operation. With the present invention, the intake of slipstream water is modulated dependent upon the speed of boat operation, by means of a pressure responsive panel placed substantially aft of the ram intake leading edge to open into the propulsion water intake passage to the jet pump. That is, the said panel is sensitive to any pressure imbalance so as to allow increased volume flow during low speed conditions. However, as the boat accelerates the said pressure imbalance diminishes and eventually disappears so that the panel is tightly shut so as to close the augmenting opening, whereby ram water only is admitted to the propulsion water intake passage via the aforesaid ram-scoop of optimized area for efficient high speed boat operation. Consequently, there is a minimum drag while providing the required pressure recovery for said high speed operation.
The boat hull is slotted so as to provide a flush opening intake for propulsion water, the slot being characterized by a rearwardly widening tunnel increasing in height as it inclines rearwardly to a rectangular opening at an interface between the boat hull and intake into the hydrojet pump unit. Characteristically therefore, the pump unit intake is rectangular into a plenum that extends rearwardly and around an impeller shaft to fair into a round flow passage. It is the first mentioned widening tunnel and second mentioned faired plenum which are to be modified by the ram-scoop of the present invention, it being an object to restrict the widening of the hull tunnel with a commensurate restriction of the interface opening at the pump unit intake, while fairing the ram scoop into the said plenum for efficient flow of propulsion water therethrough. In practice, the ram-scoop is fastened to both the boat hull and to the hydrojet boat unit, and bonded integrally into working position with epoxy resins or the like which can be made fair with both the hull and said pump unit.